Add docs for custom file input's text and $custom-file-text

[skip sauce]

docs/components/forms.md

@@ -812,3 +812,22 @@ Here's how it works:
 - We declare a `height` on the `<input>` for proper spacing for surrounding content.
 
 In other words, it's an entirely custom element, all generated via CSS.
+
+#### Translating or customizing the strings
+
+The [`:lang()` pseudo-class](https://developer.mozilla.org/en-US/docs/Web/CSS/:lang) is used to allow for easy translation of the "Browse" and "Choose file..." text into other languages. Simply override or add entries to the `$custom-file-text` SCSS variable with the relevant [language tag](https://en.wikipedia.org/wiki/IETF_language_tag) and localized strings. The English strings can be customized the same way. For example, here's how one might add a Spanish translation (Spanish's language code is `es`):
+
+{% highlight scss %}
+$custom-file-text: (
+  placeholder: (
+    en: "Choose file...",
+    es: "Seleccionar archivo..."
+  ),
+  button-label: (
+    en: "Browse",
+    es: "Navegar"
+  )
+);
+{% endhighlight %}
+
+You'll need to set the language of your document (or subtree thereof) correctly in order for the correct text to be shown. This can be done using [the `lang` attribute](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/lang) or the [`Content-Language` HTTP header](https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.12), among other methods.